Groundwater is water that has drained through surface layers of soil and rock until it reaches a layer of rock material through which it cannot pass, or can pass only very slowly. This results in the accumulation of water in the rock layers above this impermeable layer. The water is stored in gaps in the rock, or between the particles of which the rock is composed. Rock which retains water in this way is called an aquifer.

Rock types that can hold water include chalk, limestone and sandstone. Rock types which do not hold water but can prevent water moving out of a permeable rock type include granite, basalt and mudstone. Groundwater supplies springs and wells and when there has been a period with little rain, it is often groundwater that maintains flow in rivers rather than surface drainage from land.

A close look at the rocks exposed in road cuts and along streams will show the types of openings in which ground water can occur. Especially noticeable in bedrock exposures are spaces between layers that can extend for miles-the void spaces between rock particles contain water that percolates into these spaces between the layers. In most sand and gravel deposits, water occupies and moves freely within granular material.